This invention relates to amplifiers, and more particularly, to a current amplifier for a telephone line. The invention is particularly applicable for making a line amplifier part of an integrated circuit providing coupling between a MODEM and a telephone line.
In known devices in the prior art, a telephone line transformer is used to isolate a telephone line from the outside world. However, this is both cumbersome and expensive since the transformer must be extremely linear and have an isolating voltage of several thousands volts. A second approach according to the prior art is to use optocouplers, for which the linearity has been improved by complex electronic devices. These devices are expensive and consume electrical energy, which is very limited on a telephone line.
A third approach consists of using a transformer to transfer the power supply from an external source. Again, this approach is expensive since a transformer and a DC/DC converter have to be added. A fourth approach known in the prior art consists of powering the MODEM directly on the telephone line. This approach has a disadvantage in that the power supply voltage varies with the line modulation, hence the need to have a very good power supply rejection of the analog parts powered directly by the line (A/D and D/A converters). In addition, the consumed current must be very strictly controlled so that it does not disturb or create harmonics or noise on the telephone line so that an anti-echo device can fulfill its role, which excludes CMOS type logic circuits.
The first two approaches are based on an analog transfer of data between the telephone line and the outside world. The last two approaches incorporate D/A and A/D converters at the telephone line end so that the transfer is done digitally. This eliminates the linearity requirements for the insulation barrier between the telephone line and the outside world. Therefore, optical barriers, transformers or even compact and inexpensive capacitors can be used.
However, there is a need for powering converters from the telephone line without disturbing the line with distortion or noise. Finally, devices according to the prior art cannot be utilized on a long telephone line (i.e., 4-8 km), and are not compatible with high throughput transmission modes (i.e., V34).
An object of this invention is to provide a current amplifier for a telephone line capable of taking electrical energy from the telephone line to power logical and analog functions for processing MODEM transmissions and reception signals. This includes D/A and A/D converters and other telephone functions defined by industry standards (i.e., continuous characteristic, impedance summary, anti-echo).
The current amplifying device comprises first and second amplifiers installed in cascade. The second amplifier functions as a voltage follower to the first amplifier. That is, current from the second amplifier is subtracted from the current in the first amplifier. The second amplifier also functions as a rectifier or as a rectifying amplifier.
In this device, the second amplifier acts as a rectifier and the first amplifier filters harmonics generated by the second amplifier. If applied to a telephone line, the first amplifier uses the line itself as a power supply source, and the second amplifier functions as a rectifier and uses the first amplifier as a power supply source.
If the harmonic content obtained is not sufficiently low for the planned application, a third amplifier is added. The third amplifier is also installed as a voltage follower so that the current in the first amplifier originates from this third amplifier, or is subtracted from the current in this third amplifier. When applied to a telephone line, the third amplifier uses the line itself as its source. The first amplifier uses the third amplifier as its source and the second amplifier acts as a rectifier using the first amplifier as its source. This circuit gives a very low harmonic content, and satisfies the most demanding applications.
Amplifiers with a gain-bandwidth product better than 10 MHz and an open loop gain exceeding 60 dB, are capable of providing a harmonic content of less than xe2x88x92100 dB on a telephone line to which the device according to the invention is connected. Furthermore, if the minimum operating voltage of each of the amplifiers is less than a few volts (i.e., 1.8 or 2 V), the device is capable of operating with a much lower line voltage than devices disclosed in the prior art.
According to one particular embodiment, the second amplifier shunts its output current on a power supply terminal through a MOS transistor. The second amplifier also comprises means for blocking this transistor and enabling it to operate as a rectifier, or a differential amplifier, the output of which controls the gate of the MOS transistor. The source of this MOS transistor is connected to an input of the differential amplifier. Furthermore, means may be provided to bias the body of the MOS transistor at its source when its source and drain electrodes are subsequently swapped as a result of polarity inversions due to operation as a rectifier.
According to one embodiment, the second amplifier shunts its output current on a power supply terminal through a MOS transistor and comprises a differential amplifier followed by another differential amplifier, the output of which controls the gate of the MOS transistor. The source of this MOS transistor is connected to the inverting input of the differential amplifier. Also included are means of blocking the MOS transistor and making the second amplifier operate as a rectifier, and means of correctly biasing the MOS transistor body when its source and drain electrodes are swapped as a result of polarity inversions due to operation as a rectifier. The first amplifier and possibly the third amplifier may include a differential amplifier, a transistor installed as a common emitter, and a voltage follower device.
The invention is particularly applicable to manufacture of a telephone installation device, such as a device like that described above connected to a telephone line. In this case, the device or circuit according to a particular embodiment comprises an input terminal E into which an input current IE is applied, an output terminal S that absorbs the current IL from a telephone line, and a third terminal for outputting a power supply voltage VCC and current ICC. This circuit also includes a first and second amplifier installed in cascade as voltage followers, each amplifier comprising a resistance at the input (Rxe2x80x21, Rxe2x80x22) and a resistance at the output (R1, R2). The resistances Rxe2x80x21 and Rxe2x80x22 are connected in series and carry the input current IE. The current gain in each of the amplifiers is determined by the ratios Rxe2x80x21/R1 and Rxe2x80x22/R2 and the current in the second amplifier is subtracted from the output current in the first amplifier. The output current IL is kept proportional to the input current IE (it is equal to the ratio Rxe2x80x21/R1), and the second amplifier acts as a rectifier. Its current ICC is limited by the fraction of the output current IL, determined by resistance ratios Rxe2x80x21/R1 and Rxe2x80x22/R2.
The device or circuit can also comprise a third amplifier installed as voltage follower and in cascade with the two previous amplifiers. This amplifier comprises a resistance at the input Rxe2x80x20 and a resistance at the output R0. The resistances Rxe2x80x20, Rxe2x80x21 and Rxe2x80x22 are installed in series and carry the input current IE. The current gain in this third amplifier is determined by the ratio Rxe2x80x20/R0 and the first and second amplifier currents are subtracted from the current in this third amplifier. The output current IL is still kept proportional to the input current IE (Rxe2x80x20/R0 ratio).
Another object of the invention is an integrated circuit comprising a device like that described above.